Video: Adaptation of Plants in an Aquatic Habitat

In this lesson, we'll be learning about some of the most useful adaptations plants have to help them live in an aquatic environment. We'll cover adaptations to help with gas exchange, acquiring sunlight, balancing salt, and reproduction.2019-01-11

In this lesson, we'll be learning about some of the most useful adaptations plants have to help them live in an aquatic environment. We'll cover adaptations to help with gas exchange, acquiring sunlight, balancing salt, and reproduction.

Aquatic Plants

Picture an area you've been to with lots of plants. Most likely, you're picturing a forest or a grassland. However, many lakes, rivers, and streams contain just as many plants beneath the surface. Plants growing in or under water are called aquatic plants. Floating gently in the current, or anchored to the bottom, aquatic plants serve an important job for all living things: providing food and oxygen for aquatic ecosystems.

Even though floating around in a body of water might seem easy to us, it's a difficult life for a plant. Plants need special adaptations to exchange gases, reproduce, and maintain a balance of salt and water. Today, we're going to look at the structures and biochemical changes aquatic plants have evolved to help them survive in their submerged lifestyle.

Gas Exchange

All living things need to exchange gases with their environment. As humans, we breathe in and out to do this. Plants allow gases, like oxygen and carbon dioxide, to directly diffuse out of their leaves. However, this becomes trickier under water. Let's look at how plants manage this task.

Just like humans, plants need oxygen to make energy. Terrestrial plants are surrounded by oxygen in the atmosphere. Aquatic plants, however, are not. Oxygen levels are naturally lower in water, and even though plants produce their own oxygen through photosynthesis, murky waters, turbidity, and cloudy days can all curtail photosynthetic activity.

Aquatic plants have evolved a few strategies to get around this problem. First, many aquatic plants have aerenchyma tissue, a spongy network of cells that creates air spaces in the plant. The air spaces act like tunnels, allowing plants to transport oxygen from the surface to other parts of the plant. Thus, even if there is low oxygen content under water, aquatic plants are able to ship in oxygen from the atmosphere.

Cattails are aquatic plants that have 50% of their root system made up of aerenchyma tissue to transport oxygen

Aerenchyma also allow for greater buoyancy in water. The amount of gas in a plant's stems and leaves acts like a floatation device, giving them structure and support without the tough bark or wood of terrestrial plants.

In order to do photosynthesis, plants need carbon dioxide and sunlight. Terrestrial plants get carbon dioxide from the air, but in water carbon dioxide diffuses 10,000 times slower, creating a challenge for aquatic plants. Fully submerged aquatic plants have developed a unique adaptation to get around this problem using bicarbonate instead of carbon dioxide. Bicarbonate is a common molecule in water broken down to release carbon dioxide by enzymes on the surface of, or inside, plant leaves, providing a source of carbon dioxide when the gas itself is scarce.

Some plants also recycle the carbon dioxide produced by cellular respiration in the roots. Carbon dioxide is usually a waste product for cells, and humans exhale it, but aquatic plants can transport the carbon dioxide back to the photosynthetic leaves through their aerenchyma to be used in photosynthesis.

Sunlight

In addition to carbon dioxide, plants also need sunlight to do photosynthesis. In terrestrial life, sunlight is plentiful, and, unless there are clouds, sunlight is rarely blocked by the air. Under water is a different story, however. Water slows down the speed of light and blocks it from reaching aquatic plants. The deeper the plants grow, the more problematic it is to get sunlight.

Plants start by making more of the molecule that captures sunlight, chlorophyll. Chlorophyll is concentrated inside plant cells in chloroplasts. Aquatic plants make sure the chlorophyll-loaded chloroplasts are near the surface of the leaves where they can easily access the sunlight. Terrestrial plants have chloroplasts concentrated deeper in their leaves.

With limited sunlight, aquatic plants have to make the most of their leaves. Many aquatic plants grow thin, ribbon-like leaves to create a high surface area to volume ratio. That ensures the most number of cells in the leaves are able to do photosynthesis.

Java ferns have thin leaves to make the most of sunlight collection

Salt Balance

Everything needs the right balance of salt and water. If plants are submerged in water with too high of salt concentrations, the salt can enter the plant and damage internal structures. This produces a problem for marine aquatic plants. Oceans and wetlands can have extremely high salinity. How do these plants prevent internal damage from the salt? The answer is osmoregulation, or strategies to maintain a balance of salt and water.

Some plants allow the salt water to enter their roots, but then pump it back out later. These plants are called salt-secretors, such as the Api-api mangrove trees. Some plants filter the water before it enters. These plants are called ultrafiltrators. Instead of letting all the salt water in, their tissues only let water and certain ions in, preventing an influx of salt that could damage the plant. The Oriental mangrove trees in Australia, Southeast Asia, and Africa use this strategy.

Mangrove trees have special adaptations to live in salt water

Reproduction

On land, birds, bees, and butterflies take pollen, or plant sperm, from flower to flower, pollinating them and allowing them to reproduce. Sexual reproduction poses a challenge in water, since there are no pollinators to spread the pollen. Luckily, aquatic plants are highly skilled at asexual reproduction, where one plant can simply break apart into a new plant without combining sperm and eggs. For example, elodea plants fragmented by human activity don't die, but rather separate and grow entirely new plants. Some plants do try to use sexual reproduction where male plants release their gametes into the water, hoping that they sink to a receptive female plant.

Lesson Summary

Let's review.

Aquatic plants have evolved aerenchyma tissue to transport oxygen from the surface to the roots, recycle carbon dioxide from cellular respiration to do photosynthesis, and keep the plant buoyant in water.

Aquatic plants also use bicarbonate, which is more plentiful under water, as a carbon source.

They increase the amount of chlorophyll, concentrate chloroplasts near the surface, and have ribbon-like leaves to increase the amount of sunlight collected.

Using osmoregulation such as ultrafiltration or salt-secretion, plants living in high salinity regulate their water/salt balance.

Many aquatic plants use asexual reproduction, since there are few pollinators under water.

Summary:

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